This invention relates generally to hydraulic systems in agricultural vehicles and more particularly it relates to a hydraulic regulating device for a working cylinder of a hydraulic lifting unit in a farm tractor. The regulating device of this invention includes a working hydraulic circuit for controlling the working cylinder, the circuit being constituted of a working fluid pump, a tank, a directional multiway control valve provided with a sliding spool arranged between a working conduit leading to the working cylinder and the pump, the sliding spool being spring biased to be normally in the lowering position in which the working conduit is connected via a return conduit to the tank, and from the other end is controlled by a hydraulic setting cylinder.
A regulating device of this type is known for example from the German Publication No. 26 34 530 describing a hydraulic sensor coupled to the lower guide rod of a three-point linkage to act hydraulically upon an adjustment device which operates as a sequential controlling unit for controlling via mechanical rods a control spool of a directional multiway valve. The disadvantage of this known regulating device is in the fact that a relatively expensive setting or adjustment unit is necessary and moreover the control circuit has to be provided with two pumps and the linkage engages a purely mechanically operating member for setting the actual pressure values. As a consequence, the prior-art regulating device of this type is relatively expensive. In addition, the complicated processing and conversion of the signals results in a relatively inaccurate mode of operation and the entire system is sensitive to vibrations. The setting unit is designed and adjusted exclusively for the regulating device of this kind and therefore commercially available series produced structural units with sliding spools cannot be applied. This prior art setting unit has a sliding spool provided with a throttle region in the form of a land on the spool and is sensitive to foreign particles. The primary disadvantage of this regulating prior-art device arises, however, from the fact that in the case of failure of the pump which delivers control oil for the setting or adjusting unit, the regulating valve automatically switches over to its lowering position in which the lifting unit unintentionally lowers and may cause an accident.
From the German Publication No. 12 76 959 is known another regulating device for the lifting unit of a farm tractor in which hydraulic means for setting the actual value and the nominal value of the pressure are provided to act hydraulically on one side of the spring biased control spool of a directional multiway valve. It is true that this prior-art device employs only a single control conduit for controlling the directional multiway valve and the signals are processed purely hydraulically. Nonetheless it still has the disadvantage that the two setting means are interconnected by a hydraulically controlled linkage. Due to this hydrostatic processing of signals the resulting regulating device is susceptible to interference and is not much suitable for practical operation.